1. Field of the Invention
The present invention relates to a sheet-fed printer employed for an offset press, more particularly to a sheet transferring mechanism which transfers a sheet between a plurality of printing units so as to perform multiple color press on a single surface of the sheet and a single color press on both surfaces of the sheets.
2. Description of the Related Art
Printed matters such as pamphlets and catalogs are mostly printed using offset presses. The offset press is capable of multi-color printing and also well suited for mass printing. Sheet-fed printers, to which printing paper sheets of the same size are fed one by one to carry out printing, are widely employed in such offset press. In a mode where printing is to be applied on both surfaces of the sheet (double-side printing) is to be carried out in such type of sheet-fed printer, the sheet must be reversed and transferred.
As shown in FIGS. 13(a) and (b), the offset press is provided with a feed cylinder 100 and a transfer cylinder 101, with a supply cylinder 102 being disposed between and adjacent to these cylinders 100 and 101. The supply cylinder 102 has a diameter twice as great as those of the other cylinders 100,101. The printing sheet Pa subjected to a first printing is forwarded from the impression cylinder (not shown) to the feed cylinder 100 with the printed surface facing toward the cylinder 100.
Subsequently, the sheet Pa retained on the transfer cylinder 100 is grasped at its front edge by the grippers of the supply cylinder 102 and shifted to the outer surface thereof. The sheet Pa is retained on the supply cylinder 102 with the printing surface facing outward.
In a mode where printing is to be applied on a single surface with two colors (single-side printing) is to be carried out, as shown in FIG. 13(a), the front edge of the printing sheet Pa retained on the supply cylinder 102 is grasped by a plurality of holders 103 arranged in the axial direction of the transfer cylinder 101. The sheet is reversed and then fed to the transfer cylinder 101 with the printed surface facing inward. Meanwhile, when the printing sheet Pa is to be printed on both sides thereof, the rear edge of the printing sheet Pa retained on the supply cylinder 102 with the printed surface facing outward is grasped by the holders 103 and fed to the transfer cylinder 101 in the same posture, as shown in FIG. 13(b).
Accordingly, when the printing mode is switched from single-side printing to the double-side printing, the holders 103 of the transfer cylinder 101 must grasp the rear edge of the printing sheet Pa. Thus, when another printing sheet Pa having a different size is used, the phase of the transfer cylinder 101 relative to the supply cylinder 102 must be adjusted so as to allow the holders 103 to securely grasp the sheet Pa.
The phase adjustment of the reversing mechanism at switching between single-side printing and double-side printing has conventionally been carried out by changing the phase of the input gear 104 and that of the output gear 105, which transmit the driving force of the main motor, as shown in FIG. 14. Namely, the bolts 106 fastening the input gear 104 are loosened to turn the input gear 104 according to the scale. The phase of the input gear 104 is adjusted such that the holders 103 of the transfer cylinder 101 may securely grasp the rear edge of the printing sheet Pa.
Since the phase adjustment of the reversing mechanism used to be carried out manually as described above, the mode switching from the single-side printing to the double-side printing or vice versa incurs a considerable loss of time, and further a number of defective prints are liable to occur due to mishandling in the switching operation.